If you don't fill your tank up all the way when you fuel-up, you can now check This is a partial fuel-up.

You'll find the partial fuel-up option on the Add a fuel-up form just above the Fuel-up! button. This will tell Fuelly you didn't fill your tank all the way and it'll be reflected in your stats. Here's a little more info about partial fuel-ups.

We've found a big cause of wildly inaccurate mileages is partial fuel-ups. The system was assuming each fuel-up was filled completely each time. You can also go back through any old fuel-ups that were partial, click edit, check the partial box, and save. That should even out any problems the partial tanks were causing.